Someone asked about something like this on Pelican, and I've been thinking about writing this for a while. I'll post it here since we have a 'how-to' section. At the moment, its unfinished so I will be adding things later on. In the mean time, I hope this helps you figure out how to mix and match parts to build the best engine for your 924S or 944!

Since creating this thread, I've also been making videos! So be sure to check out my youtube Channel!

In the past decade, I have built numerous engines and thought I'd share some of the things that I've learned. In this guide I will be covering things that aren't usually covered in other manuals.

Now, if you are building a 2.5 liter engine, it is important to know which engine you have before going any further. Next to the bell housing on most engines is a stamp. This will tell you what year your engine is. On 1988 and later, the stamp is down next to the exhaust header.

Overview
The 944 engine was pretty high-tech for the late 1970's when it was being developed. While it was a conventional inline-four, it sported a liner-less aluminum block, hydraulic engine mounts, Bosch Motronic engine management, and balance-shafts. While inline-fours are great for fuel economy, it was tough to sell large displacement one at the time. Big inline-fours had a reputation for being harsh so consumers often stayed away! At the time, they were buying smaller Japanese inline-fours with great economy and less vibration!

During development Porsche built several test mules with various engines configurations. Eventually, they chose the inline-four layout. Despite all their flaws, inline-fours get great fuel economy due to fewer pistons causing friction inside the engine. So it was clear from the beginning that this wasn't a luxury car or Grand Tourer. Luxury cars have at least eight cylinders so that the power-strokes overlap and the crank turns smoothly. An inline-four only has a power-stroke once every 180 degrees of crank-shaft rotation. This gives the pistons enough time to come to a complete stop before the next one fires causing the crank-shaft to jerk as it speeds up and slows down. This results in a pulsating power delivery that is very noticeable, when letting the clutch out slowly at low revvs or when going up a hill in a higher gear. This vibration isn't acceptable in Luxury or Grand Touring cars, so they use V8 or V12 engines for comfort, at the cost of economy.

Sports cars on the other hand aren't really known for their refinement, so this wasn't a huge issue. However, this gap in the power-strokes also results in a poor idle that violently shakes the car. This vibration isn't really acceptable in any automobile, much less a Porsche. To over come this, Porsche developed hydraulic engine mounts to reduce the vibration felt by the passengers at idle. While the idle isn't anywhere near as smooth as a luxury car, it was noticeably better than other large inline-fours.

At the time, the largest production inline-four was the 3.2 liter Trophy-4 found in the Pontiac Tempest. Despite having plenty of power and great fuel economy it wasn't fondly remembered due to these vibrations. There were many much larger engines like the 5.9 liter inline-four that was in the Stutz Bearcat and the Fiat 29.5 liter racecar! So 2.5 liters wasn't all that large. In fact, all of the big three car manufacturers were building 2.5 liter inline-fours at the time that didn't have any sort of hydraulic mounts or balance-shafts. Instead of balancers, all of these past engines relied on soft rubber mounts to conceal the vibrations from the passengers, which wasn't easy to do. While a single piston moving up and down like in a V8 can be often hidden behind soft mounts, inline-four and straight-six engines have pistons that move together in pairs. This doubles the strength of the vibration making it harder to conceal. Mechanical vibrations that aren't isolated will transmit to the cabin causing nausea and car sickness.

In theory, an inline-four should be well balanced, however the two pistons moving up don't move at the same speed as the two moving down. This causes a non-sinusoidal imbalance that usually results in structure-borne sound and vibration traveling through the passenger compartment. In the past it was tolerated, but after the complaints with the 924 Porsche decided to license Mitsubishi's "Silent-shaft" technology to help alleviate this.

Mitsubishi's 'Silent-Shafts' are different than conventional balance-shafts. While many motorcycles and car manufacturers might have used a single balance-shaft to reduce this vibration, a single-balancer only moves the vibration from the vertical plane to the horizontal plane. Since horizontal vibrations don't really transmit to the chassis, it often got the job done. However, the Silent-Shafts are set on the outside of the engine at different heights to further reduce moments of inertia at a specific rpm range. They also cancel out the vibrations caused by each other. This means that on the highway the 944 engine feels as smooth as a straight-six and interior noise is reduced by 20 decibels. That's not considered luxury level refinement, but much better than other large inline-four cylinders at the time.

Of course, that's mostly because some manufactures didn't put balance-shafts in their four-cylinder engines until the 1990's. Even Mitsubishi who developed the 'silent-shafts' rarely used them; perhaps due to cost. The shafts used by most manufacturers today are cheaper and often packaged down in the oil pan. The 'silent-shaft' design is more sophisticated and offer far more refinement for the passengers. However, it must be said all balance-shafts are best described as vibration dampers. They are sinusoidal and reduce vibrations only at a specific rpm range, while the inherent vibration is non-sinusoidal and increases in orders of magnitude with rpm. So at high engine speeds this vibration is still very noticeable.

When introduced, the 944 engine was very well received and considered to be state-of-the art. Porsche planned to put this new engine in the 924 shell and forget the buzzy old 2.0 liter ever existed. However, just as the everything was set to go into production, they decided to widen the body of the car and call it the 944 instead. Unfortunately, since this car was supposed to be a 924, it meant the new model was left with the narrow suspension and rather spartan interior. They originally planned to have both the 924S and 944 in European dealerships by the fall of 1981.

The 924S (946) was to be a stripped down de-tuned version of the 944. After realizing there was very little to choose between the two models, Porsche decided to wait until 944 had its own interior and more options before introducing the 924S. The 944 didn't get its new interior until mid-year 1985, later than anticipated due to a metal workers strike. This threw off the arrival of both the 944 turbo and the 924S.

While the ROW version of this engine put out a respectable 156 SAE net horsepower, the US models were more focused on emissions and fuel economy. The US market 944's received an overdrive 5th gear and only produced 143 SAE net. Overtime, there were many improvements made to the engine and power increased. By the time the 2.5 liter reached the end of production in 1988 it produced 158 SAE HP in every market. However, complaints about the pulsating power delivery and how the balance-shafts don't work well enough at higher rpms started to fill the pages of automotive magazines. While it continued production in 3.0 liter form until 1995, the love affair had ended and reviewers had lost their taste for the the big four. Japanese sports cars were a fraction of the cost and had much smoother engines. So its hard to say if we'll ever see another Porsche developed inline-four again!

The early US Spec engine. 1983-1985.1
These use the same pistons that all US cars used from 1983-1987.
However, the early head gave these engines 9.5:1 compression.
They also use forged rods that are desirable.
The cam has slightly less exhaust duration than the late cam.
These engines use an eccentric roller to tension the timing belt.
These use a button style temp gauge sender. Later senders won't swap into this block.
The oil pump is smaller.
The early balance-shaft design means the covers aren't interchangeable with the later that have the turbo oil feed.
These engines have a cross-drilled crank.
This block cannot use the later one piece Oil Pressure Relief Valve (OPRV). These use a three piece OPRV and the rare early one piece.
Throttle cable adjusts at the firewall.
These engines use a 5 rib belt for accessories and 95w alternator.

Electronics
These engines use Motronic sorftware ML-1.2. The DME from these cars are best matched with the early Air Flow Meter (AFM) and injectors. It also uses an Auxillary Air Valve (AAV).

The late US Spec engine. 1985.2-1986
These engines use the same Pistons as the early US spec engine.
However, they use cast rods.
Longer duration cam introduced.
Beefier 115 amp Alternator introduced with 6 rib belt.
Temperature sensor with warning light.
Improved radiator.
Larger oil pump.
The crank is no longer cross-drilled.
The balance-shaft housing has been improved and has an oil feed for a turbo.
A smaller starter was used.
A shield was added to the Speed sensor to prevent starter interference.
The cast headers are replaced with tubular headers.
Throttle cable adjustment nuts moved towards the throttle body.
Two blade temp sender used, one for the gauge and other for the warning light.

Electronics
The later engine uses Motronic software ML-3.1. The Auxiliary Air Valve (AAV) is gone and the DME is best matched with the Air Flow Meter (AFM) and fuel injectors they are designed to run. These engines respond fairly well to chips which yield a few extra ponies.

1987
New head bumped compression up to 9.7:1
Power output was bumped 143 to 147 (SAE).
There was a small bump in torque from 140 to 143 ft/lbs.
Balance-shafts improved and use different bearings.
Cam belt auto-tensioner introduced.
Tubular headers were reinforced to prevent cracks.
One piece Oil Pressure Relief Valve (OPRV)
Improved oil cooler housing.
Oil level sensor introduced.
Exhaust 'sniffer' pipe added.

924S
The 924S is only considered to be de-tuned in Europe where their 944 had 156 hp
It has the same engine found in the 1987 US spec 944
924S engines up until 'H 01919' used the 3-piece OPRV
Also they didn't receive the auto-tensioner until mid-year
It uses the early 95 amp alternator but uses a 6 rib belt.
The 924S uses its own special temp sensor with only one terminal.
Early 1987 models have the 951 clutch line routing while later cars are routed like the early 944/931 line.

Trivia: The pre-production 924S used the early 944 dash with yellow gauges and silver fascia. There are several magazines that mention the 924S having the "944 dash" but had no pictures so people incorrectly assumed they meant the oval dash.
Pre-production 924S with original 944 fascia

That said, there were a few 924S test mules built after the 924S production ended that had an oval dash. These cars were simply testing the new 2.7 liter engine.
Post-production 924S test mule

1988 944 and 924S
New pistons increase compression from 9.7:1 to 10.2:1.
Venturi was deleted and an electronic valve for the crankcase breather was added.
The engine stamp was moved down next to exhaust header.
Resonator added to exhaust.
Larger muffler.

Electronics
The DME now contains all 8k x 8 memory in the EPROM. Software was tweaked to extract 11 more horsepower and bring the torque up from 143 to 155 ft lbs. With 158 hp on tap, these engines are definitely more potent than previous years.

The Euro spec engines
These engines had completely different pistons with 10.6:1 compression.

Electronics
The Euro engines used Motronic software ML-1.1 which had no provisions for lambda. US cars ran ML 1.2, so if you are building a European spec engine, you must use a Euro DME or use an impedance adapter.

Note: The 163 HP figure that is often thrown around is not an SAE net rating and not comparable to SAE net numbers. Often these tests are done without accessories and other tricks in order to inflate numbers, which isn't legal anymore.

Last edited by edredas on Fri Apr 07, 2017 10:35 am; edited 27 times in total

As I mentioned earlier, the 8 valve head changed around 1987 which increased compression to 9.7:1. I made the chart below to help you to better identify the heads. Note that the earlier head has a triangular shape at the spark plug, while the later is more squarish.

As you can see, the valves are well spaced and the head isn't prone to cracking between them. Since this isn't common you may just want to replace the head if you notice any of these things. Alternatively you could have the head pressure tested and repaired at your local machine shop.

Example of a refreshed 8 valve N/A head sitting in my shop.

Cleaning up these heads are very straight forward. I have put together a video here on how to make check your heads and clean them up before reusing:

951 head
The 951 or 944 turbo head is based off the late head but have an aluminum titanate (ceramic) exhaust port liner molded into them, mainly for emissions purposes. The catalytic converter on the 951 is located very far away from the head, so keeping more heat in the exhaust is important for catalytic light-off. More heat in the exhaust will also improves turbo response, for economy purposes. Turbocharged fleet vehicles often use these types of port liners in order to reduce annual fuel costs.

Since these liners are molded in, the exhaust ports cannot be ported. However, an N/A head will work fine on a turbocharged engine, especially if you are planning use the car in an environment that doesn't require a catalyst; such as racing.

951 head on non-turbocharged car
Using a 951 head on an N/A car might actually restrict the exhaust since the exhaust port is slightly smaller. Also, port liners are typically only used when the catalyst is really far from the head. On a N/A 944 the catalyst can heat up just fine without these port liners. Again in race environments that require no catalyst, its a bigger waste of time.

While these port liners may keep heat out of the cooling system, there is no return on it without a turbocharger and the heat will be lost either way.

Valves
The 951 head utilized sodium filled valves. When the sodium inside the valve is heated, it turns into a liquid. As the valve moves up and down it transfers the heat evenly into the valve seat and into the cooling system. This results in a valve without hot spots that is able to shed heat faster.

Removing/Replacing valves and stem seals
Removing the valves is rather straightforward. The best tool for this job is a 'C-style' valve spring compressor. The valves are very difficult to remove without the proper tool. Other tools can possibly damage valves, springs, seats, and guides.
Example of a head with bent valves from timing belt failure.

Make sure you use a heavy duty spring compressor to remove the valves like this MAC CF-17

Once the valves are removed, the seals can easily be replaced.

16 Valve heads

A 16 valve 2.5 liter head from a 944S is a direct fit onto any 2.5 liter block. However, you will need the pistons, wiring harness, and DME if you are doing a swap.

Porting
I would leave this to a trained professional. Hogging out the ports and putting them on a flow bench doesn't tell you anything and can actually be detrimental to power. I know this isn't very helpful, but it will save you from blowing money on something that doesn't work. If you really want more power from an N/A engine, go with a 16 valve head. Afterall, that's what the factory did._________________'86 944 -Garnet, Fully loaded, Koni suspension
'87 924S -Red, 300hp 951 swap
'87 924S -Red, New Project
'87 924S -White, Automatic
'88 924S -Red, Daily Driver, Bone stock
'93 Mx6 -White, Megan Racing suspension

Last edited by edredas on Sun Apr 02, 2017 1:51 pm; edited 5 times in total

The engine block is "Alusil", which is roughly 70% Aluminum and 30% silicone precipitate. The process is a trademark of Kolbenschmidt and results in an aluminum block that doesn't need any cylinder liners.

Pistons

The chart above lists the pistons used in the 2.5 liter engines with manual transmissions.

Example of the Pistons used in North America and Japan models with manual transmissions (83-87)

Example of the '88 pistons.

These are the pistons used in the automatic 944's.

Tolerance Groups

The stock pistons fall into a tolerance groups 0,1, & 2. The tolerance group of the piston is stamped at the top of the piston in most cases. The tolerance must match the tolerance of the block.

You can find the tolerance of the block stamped between cylinders near the edge of the block. (Note the 1 stamped into the block indicating tolerance group 1 pictured below)

Aftermarket Pistons

Sleeving the block will give you far more piston options. However, if you decide not to have the block sleeved, any aftermarket piston will need to be tin or ferro coated to prevent aluminium on aluminium contact.

Rings

Suitable piston rings can be made of any ferrous material but is important for the rings to be barrel shaped. Any sharp edges on the outer edges of the rings can potentially rip the exposed silicon crystals out of the aluminium matrix.

Honing

Alusil must be honed with Sunnen felt pads and AN-30 silicon paste. This process etches away the aluminum, leaving the harder silicon exposed.

Great info. I am using the long block from the 84 and the belt drive system. Parts I am swapping-cam tower-oil pan(for the sensor)-intake manifold and injector rail(to match my fuel lines)-all wiring and electrical. I took the engine completely apart to clean and reseal everything. Too many leaks. Thanks Paul. BTW thanks for the wheel tip. I am looking for a set of 7" phone dials for the rear. I will get them sand blasted then sand and polish them. Thanks again._________________87 924S

Thanks guys, I have a lot more that I'm going to post here; everything from vacuum diagrams to installing bearings. I've just been a little busy lately, but I plan to get back to this project soon. I have a spare engine that I plan to build from the bottom up so I'll be posting every detail with lots of pics.

Peter, that would be awesome! I'm assuming that you would import from the UK? The market is vastly different than the US where you can buy a clean 924S for pennies. In that situation, I would say get the cleanest 924S that you can afford. You won't be disappointed with any of them. That said, the '88 models really are a bit more fun to drive and the special editions are very cool. However, I would take a clean '87 model over a worn '88 without even thinking twice._________________'86 944 -Garnet, Fully loaded, Koni suspension
'87 924S -Red, 300hp 951 swap
'87 924S -Red, New Project
'87 924S -White, Automatic
'88 924S -Red, Daily Driver, Bone stock
'93 Mx6 -White, Megan Racing suspension

Again, at the risk of upsetting our brethren in the UK, no......no cars from there. Some years ago I purposely travelled there to find a choice 944S2, and came away very disappointed. Rust in sills was considered a 'badge of courage'. Yes, good ones existed but were so pricey I eventually bought back here in OZ.

You see, P-cars are, and always have been, very pricey in OZ.
EG. Last of 944S2 line in 1991 sold for $120k!!!!! And then the 968 was even dearer! No wonder so few ever made it here.
Or to put it into perspective.....a current 991 Turbo is over $400k!!!!!

So as to answer your question, no......it has to be a left hooker.....that's how God (Porsche) designed them, and thus the 30 year rule, whereby a LHD car can be legally registered in Australia (at least in my State .......some States are more lenient).

So far my initial enquiries have revealed this scenario:
Purchase price @ $4k - $5k
Transportation @ $3k
Customs Duties @ $1k
Tax on the total above @ 10% = $900
Plus local compliance @ $1k
Thus even a cheap car is not cheap by the time ALL costs are factored in. However my instinct tells me that would provide a nice driver with modern attributes but with a vintage feel. Is that not Nirvana?

Of course US cars come with Zits and Volvoesque bumpers! But as always, some things can be changed while others you learn to live with! (At least that's the mantra my ex-wifey used to chant - turns out she was wrong on all counts!)
_________________80/81 932/8 ROW

No modern LHD cars can generally be registered in Australia. They must be over 25 yo in some States, or over 30yo in my State (Qld). However, one State allows any age LHD because it has (a) military bases, and (b) huge mining projects, which brings in large populations for years at a time. These cars can be bought and sold freely within that State (WA), but must remain in that State.

Driving a LHD car was once seen to be Kool, but now there are many old LHD cars on the roads, particularly on weekends. It's only newer specialty cars that are now converted to RHD. Anyone with an older LHD car these days leaves it as such because that's how it was designed, and if one wanted to sell it back to it's country of origin, a RHD conversion would render it worthless.
Think Shelby 350 'Stang, etc.

Even factory built RHD cars are often a better drive in their original LHD guise. EG. 924/944/968 factory built RHD cars have the brake booster in the original position.......with a cross-shaft under the dash connected to the pedals on the other side!! EG. 911s up to 993 have a quaint, almost side-saddle feel about them in the RHD driver's position, as the intrusion of the wheel arch requires the pedal assembly to be shifted inwards.

Thus my desire to get something Toofahish, with modern underpinnings and older style, and usable on a regular basis. Don't get me wrong.....I love my earlier Toofahs, and I'll probably get buried in ém.
Thus a 924S........different and unique in OZ._________________80/81 932/8 ROW

Lately I have been creating videos that will hopefully show everything that I've posted here in better detail. So far, I haven't really discussed the 944 turbo but I plan to make several posts on that topic since turbocharging is a great way to add power to these engines.

Before I get started, here are a few videos on removing the engine from your 924S or 944.

The 944 Turbo was introduced in 1986 and sported a K/26 turbocharger that produced 217 SAE net HP. In 1988 a special edition called the 'Silver Rose' was introduced the K/28 turbocharger and produced 243 SAE net HP. The Turbo S models were based on the Silver Rose models and for the 1989 model year, all turbos were the Turbo S spec.

So, here's a vintage advert that shows most of the differences between the turbo and the N/A. I have personally converted a N/A car to a turbo and here are some pictures that I took. So here are just some of the parts you may want in order to turbocharge your car!

Intakes:

Throttle Cables: The turbo is shorter

Clutch lines: Technically the late 944's use the same line and route as the turbo, but if you are converting an early car you will need to re-route the lines.

Dip-sticks: The turbo stick bends around the turbo and is flexible.

AOS/Oil filler neck: The turbo models have a breather that connects to the turbo mount.

Engine Mounts: The Turbo is mounted to the left mount and oil drains through it and back into the pan.

Turbo Headers:

N/A and Turbo exhaust:

Wastegate:

Turbo and N/A Flywheel: They are the same size but turbo uses a larger 240mm disc.
R

Turbo and N/A pressure plates: The turbo pressure plate is larger and the ring gear has 2 more teeth.